|  | 5.4 | IGRP Operation
| 5.4.4 |
Describe three features of IGRP which enhance its stability
| Engineering Journal
The timers basic and no metric holddown Commands

IGRP provides a number of features that are designed to enhance its stability, including the following:

Holddowns - When a router learns that a network is further away than was previously known or it learns that the network is down, the route to that network is placed into holddown. During the holddown period, the route is advertised, but incoming advertisements about that network from any router other than the one that originally advertised the network's new metric are ignored. This mechanism is often used to help avoid routing loops in the network, but has the effect of increasing the topology convergence time.

Holddowns are used to prevent regular update messages from reinstating a route that may have gone bad. When a router goes down, neighboring routers detect this via the lack of regularly scheduled update messages. These routers then calculate new routes and send routing update messages to inform their neighbors of the route change. This activity begins a wave of triggered updates that filter through the network. These triggered updates do not instantly arrive at every network device. It is therefore possible for Device A, which has not yet been informed of a network failure, to send a regular update message (indicating that a route that has just gone down is still good) to Device B, which has just been notified of the network failure. In this case, Device B would now contain (and potentially advertise) incorrect routing information.

Holddowns tell routers to hold down any changes that might affect routes for some period of time. The holddown period is usually calculated to be just greater than the period of time necessary to update the entire network with a routing change. This can prevent routing loops caused by slow convergence.

Split Horizons - A split horizon occurs when a router tries to send information about a route back in the direction from which it came. For example, consider the graphic. Router 1 initially advertises that it has a route to Network A. As a result, there is no reason for Router 2 to include this route back to Router 1 because Router 1 is closer to Network A. The split-horizon rule says that Router 2 should strike this route from any updates it sends to Router 1.

The split-horizon rule helps prevent routing loops. For example, consider the case where Router 1's interface to Network A goes down. Without split horizons, Router 2 continues to inform Router 1 that it can get to Network A (through Router 1). If Router 1 does not have sufficient intelligence, it might actually pick up Router 2's route as an alternative to its failed direct connection, causing a routing loop. Although holddowns should prevent this, split horizons are implemented in IGRP because they provide extra protocol stability.

Poison Reverse Updates - Whereas split horizons should prevent routing loops between adjacent routers, poison reverse updates are intended to defeat larger routing loops. Increases in routing metrics generally indicate routing loops. Poison reverse updates are then sent to remove the route and place it in holddown. A router poisons the route by sending an update with a metric of infinity to a router that originally advertised a route to a network. Poisoning the route can help speed convergence.